Corrosion-resistant chromium-plated articles

ABSTRACT

In plated articles comprising a series of nickel or nickel alloy layers on a substrate, with or without a basal layer of copper or a copper alloy, and having as surface either a nickel-seal followed by a chromium layer, or a microcracked chromium layer, an intermediate copper or copper alloy layer is provided below the nickel or nickel alloy layer immediately below the nickelseal or microcracked chromium layer; this is found to cause an unexpectedly great increase in resistance to corrosion.

United States Patent inventor Peter J. Ramsden Surrey, England Appl. No. 724,340 Filed Apr. 26, 1968 Patented Oct. 26, 1971 Assignee Electro Chemical Engineering Company Limited Surrey, England Priority Apr. 26, 1967 Great Britain 19173/67 CORROSION-RESISTANT CHROMIUM-PLATED ARTICLES 7 Claims, 1 Drawing Fig.

U.S. C1 29/199 Int. Cl B32b 15/00 Field of Search 29/1966, 199, 196.3; 204/41 M Seal M UNITED STATES PATENTS Primary Examiner-Byland Bizot AttarneyCushman, Darby & Cushman References Cited ABSTRACT: In plated articles comprising a series of nickel or nickel alloy layers on a substrate, with or without a basal layer of copper or a copper alloy, and having as surface either a nickel-seal followed by a chromium layer, or a microcracked chromium layer, an intermediate copper or copper alloy layer is provided below the nickel or nickel alloy layer immediately below the nickel-seal or microcracked chromium layer; this is found to cause an unexpectedly great increase in resistance to corrosion.

, CORROSION-RESISTANT CHROMIUM-PLATED ARTICLES This invention relates to electroplating, and in particular to the production of chromium-plated articles having a high degree of resistance to corrosion.

British Specification No. 1,051,685 describes a process for obtaining highly corrosion-resistant chromium-plates articles in which the chromium layer is laid down over a so-called nickel-seal layer which is characterized by having been formed in a plating bath containing finely divided solid particles. One result of the presence in the bath of these particles is that the nickel layer formed contains minute surface irregularities, which may be, but is not necessarily, shown by a satin or semisatin finish, and it is believed that the presence of these surface irregularities gives rise to a great increase in the number of pores in the chromium coating which is laid down over the nickel-seal. The increase may be of the order of times, and is accompanied by a corresponding decrease in the area of chromium surface present for each pore. This involves a similar decrease in the cathode area per pore in the couple formed by the chromium and underlying nickel, and a corresponding decrease in the rate of galvanic attack on the nickel under the influence of moisture penetrating the pores in the chromium layer.

A similar effect is obtained if the nickel-seal/chromium combination is replaced by a microcracked" chromium layer, in which is formed a large number of very fine cracks, of the order of 400-2,000 cracks/linear inch (160-1800 cracks/cm.) each accompanied by only a small cathode area. This microcracked deposit is obtained by laying down the chromium layer under certain conditions of stress which give rise to the larger number of minute cracks in the chromium layer; it is more fully described in US. Pat. No. 3,157,585, the disclosure of which is incorporated herein by reference.

ln the production of chromium-plated articles by the above methods, it is customary to provide one or more nickel layers (which may be termed supporting layers) below the nickelseal layer or microcracked chromium layer, as the case may be, and also to provide between the substrate and the lowest nickel layer a basal layer of copper. (This layer is not always necessary, especially when coating ferrous articles, but is often provided.)

It has now been found that the corrosion resistance, for a given total coating thickness, can be still further increased to an unexpected degree by providing between the basic copper layer, or the substrate if there is no basic copper layer, and the nickel-seal or microcracked chromium layer, at least two layers of nickel, the uppermost of which is separated from the layer or layers below it by an intermediate layer of copper. More precisely, it has been found that this expedient makes it possible to increase the corrosion resistance of the coating without increasing the total amount of any of the three metals used, and indeed in some cases with a reduction in the total amount of nickel used. Thus for example in adapting a process described in British Specification No. 1,051,685, the thickness of the basal copper layer can be reduced by the thickness of the new copper layer, and the total 'weight of nickel below the nickel-seal layer can actually be reduced, without any loss, or with an actual gain, in corrosion resistance.

The invention is applicable generally to substrates comprising metals which are already electroplated in industrial practice. it is particularly important in connection with steel or other ferrous alloy surfaces, but can be applied also, for example, to substrates of zinc, aluminum, copper, brass, or plastic (synthetic resin).

It will be convenient to described the invention in more detail by reference to plated articles comprising in the plated surface a basal copper layer, two nickel layers with the second copper layer between them, and the nickel-seal and chromium layers or microcracked chromium layer. It will however by un derstood that additional layers may be provided, e.g., an additional nickel layer or layers below the second copper layer.

The copper can be employed in either or both layers as the substantially pure metal or as an alloy as known in the art; it is usually convenient that it should have the same form in each but this is by no means essential.

The nickel layers beneath the nickel-seal or microcracked chromium layer are preferably of the bright or semibright sulfur-free type. For example the basal copper layer may be followed by a semibright sulfur-free layer which carries the second copper layer and a bright nickel layer; if desired an intermediate layer of nickel of somewhat higher sulfur content, say 0.1 to 0.2 percent sulfur, may either precede or follow the second copper layer. Part or all of the nickel may be replaced by a nickel-cobalt alloy or other suitable nickel alloy in which the nickel content is at least 50 percent by weight.

Information regarding the nickel-seal layer, including a list of the various kinds of solid particles that may be present in the plating bath, will be found in British Specification No. 1,051,685, the disclosure of which is incorporated herein by reference.

The total thickness of the plate, and the thickness of the individual layers, will depend on the degree of protection, and especially the degree of corrosion resistance, which is required, but it is an advantage of the invention that it reduces the total thickness needed to obtain a given degree of corrosion resistance. Generally speaking the individual copper layers may be between about 0.25 and 10 microns and especially 0.52.5 microns thick. The nickel layers other than the nickel-seal can be between 0.25 and 25 microns thick. A nickel-seal layer will usually be quite thin, say 0.25-2.5 microns, though it can be thicker if desired, e.g., up to 5 microns. The chromium layer can be of nonnal thickness when used in conjunction with a nickel-seal layer, e.g., about 0.2-0.3 microns. To produce a microcracked chromium layer it is necessary to deposit at least 0.75 microns of chromium.

The copper and nickel layers below the nickel-seal or microcracked chromium layer may be formed by electrodeposition from conventional baths. Thus the nickel layers may be formed from baths of the Watts sulfate, high chloride, sulfamate or fiuoborate type, or any combination of them. The pH of the nickel baths may be kept between 2 and 6, preferably between 3.5 and 5.2, by means of a suitable buffer, e.g., boric acid or an acetate, citrate, succinate or fonnate buffer. The bath temperature can be between room temperature and C. or higher, temperatures of 55-65 C. being generally useful and preferred. The baths can contain agents and brighteners as described for the nickel-seal layer in British Specification No. 1,051,685.

While different baths will generally be required for forming the different nickel layers, the two copper layers are advantageously formed from baths of the same composition and temperature. We have found that when, as is the usual practice, the various baths are arranged in a circuit round which the workpiece travels, a single copper bath installation can be used. Such an installation may comprise a rectangular vessel whose length is considerably greater than its breadth, ar ranged so that, as the first stage in its path the workpiece is im mersed at one end of the vessel, and on its return journey, after passing through the desired nickel-plating bath or baths, it is immersed at the other end. Either a single more or less symmetrically or centrally placed anode, or separate anodes at each end of the vessel, may be provided. The latter arrangement is usually preferred, since it enables different current densities to be applied at each end, so that the thicknesses of the two copper layers can be individually controlled without altering the immersion time.

After its second immersion in the copper bath the work piece passes to the last nickel bath or baths, and thence to the nickel-seal and chromium baths or directly to a microcracked chromium bath or baths.

A typical circuit, omitting intermediate water rinses, is illustrated in the accompanying drawing.

The invention is illustrated by the following Examples.

copper/nickel/brass/ickel/nickel-seal/chromium The sequence of operations was as follows:

EXAMPLE 1 give copper/nickel/copper/lCKEL/nickel-seal/chromium sandwich. The sequence of operations was as follows:

1. Electrolytic cleaning.

2. Water rinse.

3. Acid dip, using 50 percent v/v hydrochloric acid.

4. Water rinse.

5. Cyanide copper plating bath, at 20 a./sq. ft. for 3 minutes.

The average thickness of the deposit was 2.5 microns.

6. Bright nickel-plating bath, at 40 a./sq. ft. for minutes. The average thickness of the deposit was 12.5 microns.

7. Water rinse.

8. Cyanide copper-plating bath, at a./sq. ft. for 3 minutes. The average thickness of the deposit was 2.5 microns.

9. Water rinse.

10. Bright nickel-plating bath, at 40 a./sq. ft. for 15 minutes. The average thickness of the deposit was l2.5 microns.

l l. Nickel-seal plating bath at 40 a./sq. ft. for 3 minutes.

The average thickness of the deposit was 2.5 microns.

12. Water rinse.

l3. Chromium-plating bath at 100 a./sq. ft. for 4 minutes. The average thickness of the deposit was 0.25 microns.

14. Water rinse.

l5. Drying.

The cyanide copper-plating bath solutions contained:

Copper cyanide 20 gll Sodium cyanide g/l Sodium carbonate 20 g/l Rochelle Salt g/l EXAMPLE 2 Zinc base die castings were plated to give a sandwich.

. Electrolytic cleaning.

Water rinse.

Acid dip, using 1 percent v/v hydrochloric acid.

Water rinse.

Cyanide copper-plating bath, at 20 a./sq. ft. for 9 minutes. The average thickness of the deposit was 7.5 microns.

Water rinse.

minutes. The average thickness of the deposit was 12.5 microns. 8. Water rinse.

. Semibright nickel-plating bath, at 40 a./sq. ft. for 15 i 9. Brass-plating bath, at 40 a./sq. ft. for 3 minutes. The

average thickness of the deposit was 2.5 microns.

10. Water rinse.

l 1. Bright nickel-plating bath, at 40 a./sq. ft. for 9 minutes.

The average thickness of the deposit was 7.5 microns.

l2. Nickel-seal plating bath, at 40 a./sq. ft. for 3 minutes.

The average thickness of the deposit was 2.5 microns.

l3. Chromium-plating bath, at a./sq. ft. for 4 minutes. The average thickness of the deposit was 0.25 microns.

14. Water rinse.

l5. Drying.

The cyanide copper-plating bath solution was that used in example 1. The brass-plating bath contained:

1. In an electroplated article having in succession l. anelectrically conductive substrate 2. at least one lower layer of nickel electroplate or nickelcobalt electroplate containin at least 50 percent nickel, 3. an upper layer of nickel eectroplate or nickel-cobalt electroplate containing at least 50 percent nickel and 4. a surface layer electroplated on said upper layer and comprising microcracked chromium or microporous chromium, the improvement which comprises interposing, between said upper layer (3) and said at least one lower layer (2) a copper-rich electroplate containing at least 50 percent copper.

2. A plated article according to claim 1 having electroplated on the substrate (1), below said lower layer (2), a basal copper-rich electroplate containing at least 50 percent copper.

3. A plated articles according to claim 1 in which the substrate is selected from the group which consists of ferrous metals, zinc, aluminum, and copper.

4. A plated article according to claim 1 in which the substrate is a synthetic plastic material rendered electrically conductive.

5. Plated articles according to claim 1, in which the nickelbased supporting layers are of the bright or semibright type.

6. Plated articles to claim 1, in which the copper-based layer is 0.25-l0 microns thick, and the nickel-based supporting layers are 0.25-25 microns thick.

7. Plated articles according to claim 1, in which both the copper-based layers are 0.5-2.5 microns thick and the nickelbased supporting layers are 0.25-25 microns thick. 

2. A plated article according to claim 1 having electroplated on the substrate (1), below said lower layer (2), a basal copper-rich electroplate containing at least 50 percent copper.
 2. at least one lower layer of nickel electroplate or nickel-cobalt electroplate containing at least 50 percent nickel,
 3. an upper layer of nickel electroplate or nickel-cobalt electroplate containing at least 50 percent nickel and
 3. A plated articles according to claim 1 in which the substrate is selected from the group which consists of ferrous metals, zinc, aluminum, and copper.
 4. A plated article according to claim 1 in which the substrate is a synthetic plastic material rendered electrically conductive.
 4. a surface layer electroplated on said upper layer and comprising microcracked chromium or microporous chromium, the improvement which comprises interposing, between said upper layer (3) and said at least one lower layer (2) a copper-rich electroplate containing at least 50 percent copper.
 5. Plated articles according to claim 1, in which the nickel-based supporting layers are of the bright or semibright type.
 6. Plated articles to claim 1, in which the copper-based layer is 0.25-10 microns thick, and the nickel-based supporting layers are 0.25-25 microns thick.
 7. Plated articles according to claim 1, in which both the copper-based layers are 0.5-2.5 microns thick and the nickel-based supporting layers are 0.25-25 microns thick. 